This is the stage of blister that you need to recognise quickly, the blister would be torn off or de-roofed and a bacterial infection will have infected the wound, the wound will be very painful and red ,swollen and angry and possibly a large red and hot area around the wound with possible tracking red lines travelling away from the blister- this is the time to get medical help as the infection can easily travel into the blood stream causing the infection to move into the leg.